1. Field of the Invention
The present invention relates to a feed rate regulator for a drill and, more particularly, to a hydraulic feed rate regulator for a portable, hand-held drill.
2. Prior Art
Modern flight vehicles are making ever increasing use of bonded composite laminates to decrease structural weight while providing strength at high temperatures. The assembly of flight vehicles requires that holes be drilled in the composite laminates to accommodate bolts and other fasteners.
When the drill bit reaches the blind or back surface of the thickness being bored, there is a surge or sudden movement of the drill bit due to the precipitous decrease in the resisting force. More particularly, the resisting force decreases approximately 50 percent (50%) when the tip of the drill bit penetrates the thickness of the part being drilled and breaches the back surface. When the part is made from metal, the sudden surge of the drill bit may cause a slight burr which can be easily removed with a file. However, when the material is a composite laminate such a surge commonly causes delamination of several plies adjacent the drilled hole.
Repair of the composite plies requires the application of an epoxy adhesive followed by the continuous application of pressure by means of clamps while the epoxy adhesive cures. The hole must then be bored again because the epoxy adhesive invariably flows into and partially fills the opening. The repair process for a composite laminate is thus a time-consuming procedure which is labor intensive, and therefore costly.
One solution to this problem is to bore the necessary holes in composite laminates with an automatic drilling machine having a controlled feed rate. Such a machine advances the drill bit a fixed distance per revolution of the drill bit. Appropriately setting the feed rate avoids surge and thus the problem of delamination adjacent the drill hole because the feed rate is a controlled constant.
However, drilling machines are very expensive, and thus require a significant capital investment. Additionally, the machines are large and bulky, requiring much floor space. Furthermore, it is difficult and sometimes impossible to use the machines to drill holes in surfaces which are not readily accessible.
The operator of a drilling machine must be trained in its use. Furthermore, its weight and bulk made it impractical to transport a drilling machine to scattered or distant locations to perform repairs in the field. The use of a drilling machine is thus typically limited to factories where flight vehicles are assembled.